In the field of communications there is a constant need for phase-locked loops (PLL) having different medium frequencies and bandwidths. In transmission networks, for example, the last data transmission functions are performed on network terminating units. The twin cable, which is several kilometers long and through which the network terminating unit transmits data to and receives data from the network, impairs the received signal. Since the network terminating unit takes its timing from the incoming signal, that is, it is synchronized and locked to the phase of the incoming data flow, a distorted incoming signal makes great demands on the clock signal regeneration block in the data receiver of the network terminating unit. The clock signal is more usually generated in a phase lock circuit, which generates a clock signal locked to a reference signal. On this principle, the data flow from the network functions as a reference signal. The phase lock circuit must be as immune as possible to any phase noise from the network, and any phase noise produced by itself must be as low as possible.
In its basic configuration the phase lock contains basic blocks according to FIG. 1: a voltage controlled oscillator VCO, a phase comparator and a low pass filter. A signal at reference frequency is brought to the first input of the phase comparator while the feedback oscillator output signal, which is also the output signal of the circuit, is brought to the second input. In several applications, such as frequency synthesizers, the feedback branch contains a loop divider with a division number which can be changed in a programmed fashion. Hereby the output signal frequency is divided before taking it to the phase comparator, whereby it is possible to form frequencies that are higher than the reference frequency but which are locked to this. The phase comparator detects the phase difference between the input signals and gives an output signal which is proportional to the phase difference and which is led to a low pass filter working as a loop filter. Its output voltage again is the control voltage of the voltage controlled oscillator. When the loop is balanced, the phase of the output-frequency signal is locked to the phase of the reference-frequency signal.
The phase lock must be such that, firstly, it preserves its balanced state and the output signal is not modulated despite any rapid variation, such as phase noise, contained in either input signal, and such that, secondly, the setting time is as short as possible when the output frequency is changed. Thus, great demands are made on the loop filter, even conflicting ones. When the loop is locked, the filter must have a low cut-off frequency, so that the output signal noise is seen as modulation in the output, while during setting, when the output frequency changes, the loop must have a high cut-off frequency for the setting time to be short. If the cut-off frequency of the loop filter can be changed by controlling, then the lock detector shown in FIG. 1 can be used for forming this control.
Several different connections may be used as oscillators proper. They may be directly voltage controlled or a current controlled connection with a voltage-to-current transducer in front very often forms the core. Suitable current controlled oscillators for use in a phase lock are, for example, ring oscillators, which are formed by connecting an odd number of either single-pole or differential inverters in a sequence, and a current controlled balanced relaxation oscillator. The latter in particular has a low gain and a narrow stable state band. These oscillators are current controlled and therefore a voltage-to-current transducer is required in front of them to convert the voltage from the loop filter into a control current.
Great demands are made on the voltage controlled oscillator in the PLL circuit of the network terminating unit. It must be
a) phase stable and have a low noise, PA1 b) the adjustment range must be wide and the gain high in a state of change, PA1 c) the gain should be low in a stable state, and PA1 d) the output frequency should be linearly dependent on the control voltage.
The costs of manufacture and assembly must also be low, and this is indeed a factor which limits the oscillator's complexity from an industrial aspect. These demands are partly conflicting: if the oscillator connection has a high gain, a wide adjustment range is achieved, but at the expense of noise, and correspondingly a low gain will produce a narrow adjustment range only. In a state of change a high gain is required for the locking to take place quickly, but in a balanced state a high gain will cause disturbing phase noise. A low gain improves phase stability and reduces any noise caused by a reference-frequency signal which has passed through the filter. So far no such oscillator has been presented which would meet all these requirements.
It is true that numerous phase lock circuits are available commercially, but most of these are dimensioned for a special application, such as radio and telecommunication etc., and thus for a certain band width. There is no such general purpose phase lock available which would be able to operate on a very wide band but which would still meet strict phase noise and band width requirements which are necessary in various fixed network applications.
Patent application EP-0495573, applicant National Semiconductor Corporation, USA, presents a ring oscillator connection made in a Complementary-Metal-Oxide-Semiconductor (CMOS) process. Since process variations in the microcircuit production bring about variations in the characteristics of P- and N-channels of transistors, the application suggests the use of a special compensating connection, which makes a voltage-to-current transducer give a very small compensating increase or reduction to the current before it is taken to work as control current for the oscillator. An increase or reduction produced by the compensating connection implemented with FET transistors maintains a frequency control range which is standard despite process variations, such as transistor threshold voltages, transconductance and source/drain capacitance. It also compensates for external variations, such as variations in temperature and in operating voltage.
Although this known connection does affect the output current of the voltage-to-current transducer before it is taken to work as the oscillator control current, it can not be used for implementing a general purpose voltage controlled oscillator with a frequency that may be chosen from a wide range, for example, from a range of 50 MHz . . . 150 MHz. To produce such a range the transducer output current ought to vary in a range of several milliamperes, while a loop filter output voltage varies within a typical range of 1.5 . . . 3.5 V. This would demand an enormous current gain from the transducer, which would result in an intolerable noise at the oscillator output.
The invention aims at a method of controlling a voltage controlled oscillator VCO intended for a phase lock circuit and at a control connection allowing the selection from a wide frequency range of a desired frequency as the oscillator's medium frequency while the frequency band in a stable state is still narrow and allowing in all conditions a low noise in the oscillator output signal despite the wide frequency range.